Sound test
Note: This requires the European or updated version of the game. Highlight the "Music" option at the "Options" menu. Press 1 to go forward, then 2 to go back. While the music is playing you can switch between "Old" and "New" to hear the music from the original version or THE New version.

Boss Attack mode
Complete the Sanctuary in Story mode.

Curly Story mode
Defeat Curly in Story mode.

Sanctuary Time Attack mode
Complete the Sanctuary in Story mode.

Alternate endings
Complete the indicated task to unlock the corresponding ending.
Bad: Leave with Kazuma after your second time through the Egg Corridor.
Normal: Complete the escape sequence without going through the Sacred Grounds.
Good: Complete the Sacred Grounds area.

Make the rocket that allows access to the last cave, but do not talk to Momorin to get the booster back. Use the rocket and keep the Mimiga Mask.

The Spur
When you defeat Curly in the Sand Zone, do not trade your Polar Star for the Machine Gun. You can eventually trade it later for the Spur, the strongest weapon in the game.

Bubbler weapon
Obtain the Jellyfish Juice, then use it on the fireplace in the Assembly Hall in Mimiga Village to the Bubbler. You can hold [Down] when it is equipped and form a total of one hundred bubbles that deflect blockable enemy attacks and cause a lot of damage because of all the sharp stars it releases. It is also useful when facing a lot of enemies, such as the hordes of birds that fly over to you and follow in the Sand Zone. Also try it when in the Labyrinth and an enemy traps you by jumping up and trying to attack. You can remain in any location and charge up the Bubbler; it will release the stars and hit the enemies for you. In the Sand Zone, use it as a shield and launch the stars. You can even use it to get the Arms Barrier. When fighting Monster X and he releases the heat-seeking fish-like missile projectiles, the Bubbler protects you and eliminates them in a single hit. You can also use it against Balfrog to block all his missile attacks.

Chaco's lipstick
Talk to Chaco about the Jellyfish Juice, then rest in her bed. You will wake up with Chaco in the bed and her lipstick in your inventory.

Curly's underwear
Collect the Map item while going through the Mimiga Village. Use the Map in the back room after defeating Curly in the Sand Zone to find a secret passage. Use the Map to reach the end of the passage, then use the "Look" command on the ledge on the left to get Curly's underwear.